I found this oddity today while examining some fine grained finds. This is basically soft limestone, where the rock is pretty soft and most of the calcite has been dissolved. I forgot to include a scale, but if I were to guess, it's about 1/2" across the structure (12.7mm). I plan on measuring again. There were several of these throughout the piece, but this was the most prominent.   My guess is some sort of Bryozoan.